This invention relates generally to high power lasers and, more particularly, to techniques for achieving high power and high efficiency in fiber lasers. Achieving high efficiency in fiber lasers has long been a goal for designers of high power laser systems employing fiber lasers. A convenient measure of efficiency is optical-to-optical slope efficiency, which is the slope of a characteristic curve plotting optical output power against optical input pump power. This curve is basically a straight line once the pump power reaches a threshold value at which lasing begins. It is known that an ytterbium (Yb) doped fiber laser pumped at 975 nm wavelength can operate at slope efficiency of approximately 80% for an output wavelength near 1080 nm. Unfortunately, achieving efficiencies this high is much more difficult for wavelengths in the “eye-safe” region above 1.3 μm. For example, an erbium:ytterbium (Er:Yb) codoped fiber laser has achieved a slope efficiency of only about 40%, limited by the pump/signal wavelength ratio of 975 nm/1550 nm (63%).
A published report has indicated more promising results for a thulium (Tm) fiber laser pumped at 790 nm, amplifying at 2000 nm wavelength, and achieving slope efficiencies grater than 50%. For the Tm fiber laser, it is possible to exceed the nominal pump/signal quantum ratio of 790 nm/2000 nm (40%) because of a known “2 for 1” Tm cross-relaxation mechanism, which promotes two Tm energy level sites to the upper laser state manifold for a single absorbed pump photon, and thereby doubles the potential laser efficiency. In addition, it has been observed that the Tm silica fiber laser efficiency is strongly dependent on the fiber temperature and, more specifically, that the Tm fiber laser slope efficiency increases from 40% to 52% when the coolant temperature is lowered by only approximately 10° C.
In spite of these promising findings reported in the technical literature, no-one prior to the present invention has produced a high power fiber laser system with efficiencies well above those reported, and preferably capable of operation at eye-safe wavelengths. The present invention achieves this goal and has additional benefits and advantages.